The fundamentals
Working principle
Flow batteries were initially developed in the 1960s by The USA’s National Aeronautics and Space Administration, better known simply as NASA. But it wasn’t until the 1980s that their popularity picked up speed, after they were proven to last for more than 10,000 charge/discharge cycles. Along with the continuously growing installed base of renewable energy systems, most notably solar and wind power, it has become obvious that the need to store large, indeed very large, quantities of electrical energy for longer periods of time is growing equally quickly. Such energy storage is essential if we are to achieve a total transition from fossil fuels to renewable energy.
The term flow battery covers a family of storage systems where each one will apply the same fundamental working principle, while using different combinations of active materials. The heart of a flow battery is a so-called electrochemical cell, which is a multi-layer assembly of an ion-selective membrane, catalyst layers and electrodes.
A complete flow battery system, also referred to as a redox flow battery or RFB, is constructed around such electrochemical cells, where chemical energy is provided by the chemical reaction of two active materials. The active materials are contained within the system, separated by the membrane, and circulate in a closed loop, each one in their own respective space.
When an electrical power source is connected, which is when the battery is charging, a chemical redox reaction starts. Ion exchange then occurs through the membrane, resulting in electric current. During discharge, when applying an electrical load, the reverse chemical reaction takes place.
The voltage of the electrochemical cell is determined by the Nernst equation and ranges in practical applications from 1.0 to 2.2 V, depending on the selected active materials.
In order to increase the total electrical power, individual electrochemical cells are stacked, which is another way to say that the cells are electrically interconnected in series. To design systems with (very) large power levels, multiple stack assemblies can be interconnected.
The power [MW] of a flow battery system, as depicted above, is determined by the surface area of the ion-selective membrane, while the capacity [MWh] of the system is determined by the volume of the catholyte and anolyte reservoirs.
The fact that the membrane surface area and the reservoir volumes can be dimensioned individually highlights one of the most distinguishing properties of flow batteries, as opposed to traditional electricity storage systems where power [MW] and capacity [MWh] scale simultaneously.
Engineered for 25 Years: Commercial Durability Proven in Elestor’s Hydrogen–Iron Flow Battery Technology
Authors:
Kaan Colakhasanoglu (Stack Research Specialist)
Wiebrand Kout (CTO)
Abstract
Elestor’s hydrogen–iron flow battery architecture is put to the test and evaluated under continuous, commercially relevant operating conditions to assess durability, performance stability, and lifetime potential. The system combines a hydrogen gas circuit with an aqueous iron-based electrolyte, enabling independent scaling of power and energy while relying on abundant, low-cost active materials (±2.8€/kWh, enable reaching 15€/kWh CAPEX and 0.02€/kWh Levelized Cost of Storage at system level).
An extended continuous cycling campaign demonstrates stable operation at practical current density, temperature, and voltage windows representative of real-world deployment. Measured performance remains stable and fully recoverable through standard conditioning procedures. The absence of structural or electrochemical failure under sustained operation provides a robust empirical basis for extrapolating operational lifetimes of 20–25 years under standard use profiles.
This work positions hydrogen–iron flow battery technology as a durable, scalable, and economically viable solution for long-duration energy storage.
Energy Independence for Islands
Authors:
Willem de Vries (Charged Islands)
Mohamad Alameh (Charged Islands)
In cooperation with Floris van Dijk (Elestor)
Abstract
Due to recent declines in the cost of photovoltaic solar generators (PV) and battery energy storage systems (BESS), baseload renewable energy systems (BRES) can now outcompete a grey generation mode (diesel electricity generation) on a 24/7 basis. BRES now promise a 30% reduction in electricity generation costs compared to diesel generators for a wide set of geographies, often reducing generation costs by 100 EUR/MWh. This gap is expected to grow with the introduction of cheaper long duration energy storage (LDES) systems in the future, potentially reducing cost of electricity supply by 50% compared to diesel generation.
With economic arguments in favour of BRES, a movement towards deployment of such systems can be expected and is also encouraged and supported by the writers of this white paper.
Numerous islands will have to overcome various hurdles though trying to implement BRES. Examples of such hurdles are shortage of development & financing capabilities as well as the shortage of land and a lock-in of diesel generation assets.
Long-term performance of hydrogen-bromine flow batteries using single-layered and multi-layered wire-electrospun SPEEK/PFSA/PVDF membranes
Sanaz Abbasiab, Yohanes Antonius Hugob, Zandrie Bornemanac, Wiebrand Koutb and Kitty Nijmeijer*ac
aMembrane Materials and Processes, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands. E-mail: D.C.Nijmijer@tue.nl
bElestor BV P.O. Box 882, 6800 AW Arnhem, The Netherlands
cDutch Institute for Fundamental Energy Research (DIFFER), P.O. Box 6336, 5600 HH Eindhoven, The Netherlands
Abstract
Sulfonated poly (ether ketone) (SPEEK), perfluorosulfonic acid (PFSA), and polyvinylidene fluoride (PVDF) were wire-electrospun. Subsequently, multiple electrospun layers in different arrangements were hot-pressed into sustainable membranes for use in hydrogen-bromine flow batteries (HBFBs). The relationship between the electrospun layer composition and arrangement, membrane properties, and battery performance was explored. Wire-electrospinning and hot-pressing improved SPEEK and PFSA/PVDF compatibility, yielding dense membranes. Higher SPEEK contents lead to rougher morphologies, while the insulating nature of PVDF decreases the ion exchange capacity (IEC) and HBr uptake compared to commercial PFSA. The multi-layer assembly negatively impacted the membrane transport properties compared to the single-layer arrangement. Although wire-electrospinning improves the polymer dispersion and fixed charge density, SPEEK-rich regions of the blend membranes lack the high selectivity of PFSA, thus reducing the ionic conductivity. This is especially clear in the multi-layer membranes with accumulated SPEEK in the intermediate layer in the through-plane direction. Following initial property comparisons, thinner wire-electrospun SPEEK membranes were prepared with area resistance in the PFSA-comparable range. Among the wire-electrospun SPEEK/PFSA/PVDF membranes, the single-layered membrane with 8 wt% SPEEK (SPF1-8; 62 μm) displayed stable HBFB performance at 200 mA cm−2 over 100 cycles (64 cm2 active area). Based on the ex-situ measurements and cell performance results, a total of ∼10.5 wt% SPEEK is suggested as the limit for both single and multi-layered wire-electrospun membranes, combined with a maximum membrane thickness of ∼50 μm. This ensures robust HBFB performance, positioning wire-electrospun SPEEK/PFSA/PVDF membranes as a PFSA alternative in energy storage.
Dutch hydrogen battery promises 2 cents per kWh and lasts for decades
Article on TW.nl: A Dutch breakthrough in battery technology could keep the electricity grid stable for decades, at low cost and as a sustainable alternative to lithium batteries. This hydrogen–iron flow battery could significantly reshape large-scale energy storage.
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Hydrogen-iron flow battery could deliver 25-year grid energy storage with 80% efficiency
Article on Interesting Engineering: A Dutch battery manufacturer has developed a revolutionary hydrogen-iron flow battery that could reportedly power grids for decades while maintaining stable efficiency across tens of thousands of charge-discharge cycles.
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Article and interview in Solar365 magazine: Elestor to build largest hydrogen battery ever
For the energy transition to succeed, sufficient renewable generation is required, but also the ability to store that energy for longer periods. Technologies capable of storing energy between eight and one hundred hours can play a crucial role. A broad consortium has received €22 million in funding from the Dutch National Growth Fund for the so-called SLDBatt project (Sustainable Long Duration Battery), which focuses on long-duration electricity storage.
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